1. Field of the Invention
The present invention relates to the investigation and analysis of vibration. It is particularly concerned with investigating vibration using a laser vibrometer and may be used, for example, to analyse vibrations of vehicles.
2. Summary of the Prior Art
It is already known to analyze the surface vibration of an object such as a vehicle using a laser vibrometer. Vibration of the object""s surface can be induced by internal machinery; for example a car""s piston engine causes external body panels to vibrate with characteristics governed by the engine rotation speed and number of cylinders, amongst others. A laser beam from the vibrometer is caused to be incident on the object, and laser light reflected from the object is detected, and its frequency compared with the light emitted from the laser vibrometer. Vibrations of the object will cause modulations in frequency in the reflected light, compared with the light generated, and this change can be analysed to investigate the vibration of the object and hence to identify it. Typically laser radars employ an optical heterodyning system to down convert the received signal from optical frequency to radio frequency (RF) for subsequent demodulation and signal processing.
For example, the article entitled xe2x80x9cDemodulation Scheme for Vibrometers and Associated Effects on Targets Classificationxe2x80x9d by B Ruck et al, published in AeroSense ""98/3380-23, SPIE Volume 3380 pages 151 to 162 of 1998, discusses the analysis of the output of a laser vibrometer. In that article, it was proposed that the light output from the laser vibrometer was modulated, to generate a modulated change in phase discussed above, and the output from the laser vibrometer was demodulated to be analysed. The article discussed the operation of that demodulation in detail.
The article by B Ruck et al discussed above assumed that the object, although vibrating, was otherwise stationery. The techniques disclosed in that article cannot be used if there is bulk movement of the object, since the light reflected from the object is then Doppler shifted by an amount proportional to the velocity of the object resolved in the direction of the laser vibrometer. The amount of that Doppler shift is large relative to the vibrations. This prevents successful results being obtained if only the techniques disclosed referred to the article referred to above are used. In this case, it is impractical to perform demodulation on the full receiver bandwidth because of the additional noise which would be included. Moving objects also introduce requirements for locating and tracking the object through space so that the laser beam can be steered onto and maintained on the surface of the object for sufficient lengths of time.
The present invention seeks to provide an apparatus and method for analysing vibrations using a laser vibrometer, in which it is possible to detect and analyse vibrations of an object independent of bulk movement of that object, At its most general, the present invention proposes that an estimate is derived of the effect of the bulk movement of the object on the signal, and then that estimate is used to determine a range of frequencies which are analysed in order to detect the vibration.
In effect, the bulk movement of the object means that signals from the object derived from the reflected light detected have a potentially wide bandwidth in which vibrations could occur, but that estimate of the movement is used to enable the bandwidth to be reduced to a manageable size.
There are many techniques which may be used to derive the estimate of the effect of the bulk movement of the object. In general, iterative methods are used in which the estimate of the current movement of the object is obtained from one or more previous estimates of that movement, which predictions are themselves derived from the signals from the vibrometer. Preferably, in order to minimise processing, techniques are used in which one estimate is derived using only the information from the immediately preceding estimate.
It should also be noted that, since the present invention is concerned with analysing vibrations of an object, when that object may be moving, it can be assumed that successive estimates of the movement will vary in a continuous way. Values such as position, velocity and acceleration will vary smoothly, rather than in a step-wise manner. This enables limits to be put on the range of possible values from one estimate to the next, simplifying processing.
Thus, the present invention may be considered be an iterative kinematic tracking and prediction arrangement, in that it makes use of iterative techniques, assumes the movement of the object follows rules of behaviour, and seeks to track the changes in movement of the object using a predictive method which is tested against the actual behaviour.
For example, the mathematical techniques known as Kalman filters may be used. At regular time intervals, measurements are made, using information derived from the vibrometer signals, which are used to modify the current estimate of the xe2x80x9cstatexe2x80x9d of the object, namely its movement. Kalman filters use a recursive algorithm, and so the complete history of the object does not need to be stored. This makes it suitable for real-time systems.
However, the present invention is not limited to the use of Kalman filters. For example, other estimates of the xe2x80x9cstatexe2x80x9d of the object may be used using the mathematical techniques known as alpha-beta tracker, path-following methods graph-theory, hidden-markov models, bayesian techniques, and stochastic resonance.
Once a suitable estimate of the effect of the bulk movement of the object has been determined to enable a suitable bandwidth to be investigated for the effect of the vibrations, then the subsequent analysis of those vibrations may be on the basis of the techniques disclosed in the article referred to above.
Thus, the present invention may provide a method of investigating vibrations of an object comprising:
generating coherent laser energy and causing some of the laser engergy to be emitted towards to object;
detecting some of said emitted laser energy which is reflected from the object;
causing other of said generated laser energy to be mixed or interfere with the laser energy reflected from the object;
generating a signal corresponding to the interference thus caused; and
analysing the signal to derive therefrom a further signal representing the vibrations of the object;
wherein the signal is analysed to derive therefrom a frequency relating to the bulk movement of said object, another signal is derived from said signal which is of restricted bandwidth relative to said signal, said restrictive bandwidth encompassing said frequency and also encompassing the frequencies of vibration of said object, and said further signal is derived from said another signal.
The present invention may also provide an apparatus for carrying out the method referred to above.
Preferably, the predetermined frequency range is of the order of xc2x110 kHz.
Preferably, the analysis to derive said successive values using iterative methods, and more preferably an iterative method in which one estimate of the value is derived using information from immediately preceding iteration, and not from information derived from earlier iterations.
Preferably, the laser light which is generated is modulated at a predetermined modulated frequency, This ensures that when the laser beam reflected from the target and the internal reference beam are mixed, there is a component at a frequency that can be processed. This additional modulation is included so that even when the Doppler shifts due to bulk movement of the object are negative and substantial, the resulting frequency output is still positive since negative frequencies cannot readily be represented by the vibrometer detector where negative Doppler shifts due to bulk movement will never be encountered, and then the additional modulation is not necessary , this may be the case when the sensor is always moving very rapidly toward the object.